Golf ball manufacturers have been using ionomer resins for golf ball component materials because of their durability, rebound, and scuff resistance characteristics. Ionomer resins are generally understood as thermoplastic polymers that are ionically crosslinked, and which may contain both hydrogen and ionic bonds. Ionomers are typically derived from copolymers of an olefin, e.g., ethylene, and an α,β-unsaturated carboxylic acid, e.g., acrylic acid, methacrylic acid, or maleic acid, that are partially neutralized with metal ions such as sodium, lithium, zinc, or magnesium ions, as disclosed in U.S. Pat. Nos. 3,264,272 and 3,404,134. Examples of commercially available ionomer resins include, but are not limited to, SURLYN® from DuPont de Nemours and Company, and ESCORT® and IOTEK® from Exxon Corporation. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
Those of ordinary skill in the art are aware that increasing the neutralization of ethylene-based ionomers during manufacturing reduces the processability of the material. This is demonstrated by the decreased melt flow index of the resulting material, which can be measured according to ASTM method D-1238, procedure A using a 2.16 kg weight. In fact, in some cases, the melt flow index of the material is decreased to the point that the material does not flow at all under normal processing conditions. Once the neutralization is greater than about 60 percent, depending upon the cation(s) present, the melt flow of the ionomer is too low to easily process the material. And, in some cases, e.g., trivalent cations, the threshold percent for non-processability of the material is much lower than about 60 percent. As a result, commercially available ethylene-based ionomers are generally only partially neutralized.
And, while certain highly neutralized polymers have recently been discussed in U.S. Pat. No. 6,329,458, U.S. Patent Publication Nos. 2001/0019971 and 2001/0018375, and International Publication No. WO 01/29129, these polymers are produced using organic fatty acid salts. Potential compatibility issues remain with these fatty acid-based highly neutralized polymers, however, due to their hydrophobic backbone moiety. In addition, the fatty acids may, depending on molecular weight and/or boiling point, vaporize during injection molding, generating a large amount of gas, which may lead to molding defects. The presence of this gas may also result in gas constituents settling on the surface of the molded object, which greatly lowers the adhesiveness of the object to paint, additional layers, or the like.
In addition, the recent trend toward light stable cover materials has introduced durability and adhesion issues, particularly between an ionomer resin inner cover layer and a polyurethane outer cover layer. For example, the inner components of most commercially available polyurethane covered or layered golf balls undergo a surface treatment, e.g., corona discharge/silane dipping, to overcome the adhesion problems. The surface treatment, however, adds cost and time to the manufacturing process.
Thus, a need exists in the golf ball art for a resin material that is easily processed with desirable melt flow and molding characteristics. In addition, a need exists for improved cover layer materials that reduce or eliminate adhesion problems with other cover layer materials. Moreover, a need exists in the art for a method to mold this material into highly durable, resilient golf balls tailorable to have virtually any combination of feel and spin rate.